Heretofore, insulin has been considered to be a metabolic and growth hormone. Our recent studies indicate that insulin is essential for milk-protein gene expression in mouse and rat mammary epithelial cells. The hormone is effective in vitro, in this repect, at physiological concentration, no other known factor can substitute for it, and its efficacy is independent of any influence on cell maintenance. Other reports suggest that insulin may also play a vital role in the development of certain other cell types. We propose that, in addition to being a metabolic and growth factor, insulin may also serve as a "developmental" hormone which is essential for selective gene expression in certain cells at particular stages of ontogeny.